You can;t flash a custom firm with itunes without using winpwn (I don;t think). Winpwn has to put the phone into a "special" dfu mode allowing you to do the custom firm. Download winpwn and try that (or whatever the Mac equivalent is called).

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I once prayed to God for an iPhone, but quickly found out He didn't work that way...so I stole an iPhone and prayed for His forgiveness.

A dog is the only thing on earth that loves you more than you love yourself. - Josh Billings